Method for enhancing the recovery of oil using steam stimulation process

ABSTRACT

In a &#39;&#39;&#39;&#39;huff and puff&#39;&#39;&#39;&#39; type steam stimulation process for enhancing the recovery of oil from an underground formation, amounts of caustic material such as sodium hydroxide are added to the steam in concentrations greater than 500 parts per million, resulting in further enhancement of recovery.

I United States Patent [191 1 3,924,683

George et al. Dec. 9, 1975 [5 METHOD FOR ENHANCING THE 3,259,186 7/1966Dietz 166/263 RECOVERY OF OIL USING STEAM 3,279,538 10/1966 Doscher166/303 X 3,434,544 3/1969 Satter et a1. 166/303 STIMULATION PROCESS3,527,303 9/1970 Zwicky 166/303 [75] Inventors: William D. George,Missouri; 3,581,823 6/1971 Feuerbacher 166/267 Chin-Wen Shen, Houston,both of Primary Examiner-Stephen J. Novosad [73] Assignee: Getty OilCompany, Los Angeles, Attorney, Agent, or FirmArnold, White & DurkeeCalif.

2 [2 Filed Oct 15, 1974 ABSTRACT [21] Appl. No.: 514,835

In a buff and puff" type steam stimulation process for enhancing therecovery of oil from an under- [52] 3.8.81. 166/303 ground formationamounts of caustic maten-al Such as [51] F t. E21B 43/24 sodiumhydroxide are added to the steam in concen leld 0 Searchtrationsygreater h parts p million resulting in References Cited furtherenhancement of recovery.

UNITED STATES PATENTS 25 Claims, N0 Drawings 3,036,631 5/1962 Holbrook166/263 METHOD FOR ENHANCING THE RECOVERY OF OIL USING STEAM STIMULATIONPROCESS BACKGROUND OF THE INVENTION This invention is directed toenhancing the recovery of petroleum oil from underground reservoirs.

More particularly, the invention is directed to such enhancementutilizing a steam stimulation process in which steam is periodicallyinjected into an underground reservoir through a well communicatingthereto.'

Steam stimulation processes in which steam is periodically injected intoan underground reservoir, then not injected, are well known in the art,and are often referred to as huff-and-puff processes. In such processes,steam is injected into a well for a certain period of time. Then, steaminjection is temporarily tenninated and well fluids including oil (alongwith water and steam) are produced from the well. Production is laterterminated and steam injection is resumed for a further period. Steaminjection and production are alternated for as many cycles as desired.

A variation of this type of steam stimulation process includes soakingthe steam in the formation following injection thereof. In accordancewith this variations, steam is injected, then the fonnation is allowedto soak with the steam therein, and then well fluids are producedthrough the well. Further cycles are then performed which include steaminjection, soaking if desired, and production.

Steam stimulation of the huff and puff type, as described above, iseffective in many instances to enhance the recovery of petroleum oil.This invention is directed to such a process which includes the additionof additives in the steam which further and importantly enhance theproduction of oil from the well.

SUMMARY OF THE INVENTION The invention relates to a buff and puff steamstimulation method wherein very small amounts of a caustic material suchas sodium hydroxide are included in the injected steam. Inclusion ofsuch additive in the small amounts contemplated by the invention hasbeen found to be quite useful in enhancing the production of oil, bothin terms of increasing the initial rate of production and in terms ofincreasing the' total amount of oil recovered.

In accordance with the invention, at least 500 parts per million causticmaterial is added to steam, and the steam is injected into theunderground reservoir through a well. After steam .(including thecaustic material) has been injected for a certain period of time, steaminjection is terminated and production of oil (including at least somewater and steam) through the well is commenced. The first stimulationcycle is completed upon termination of production, and this cycle isrepeated with as many similar cycles as desired.

Alternatively, following injection of the steam including causticmaterial, the steam may be allowed to soak into the formation. Then thecycle is completed by producing well fluids following such soaking. Thiscycle is repeated as many times as desired.

The invention also contemplates the provision of an optimum level ofcaustic concentration in the injected steam. For many contexts of use,this level has been determined to be about 2000 parts per million ofcaustic material.

2 In accordance with other embodiments of the invention, steam having ahigher caustic content is injected during the first few cycles. Then thecaustic content of the steam is reduced and the steam injection andproduction cycles are repeated as many times as desired. Here again, inaccordance with these additional embodiments of the invention, optimumlevels of caustic content are determined, and in many contexts of use ithas been found desirable to employ a steam having a causticconcentration of about 6000 parts per million for the first couple ofcycles, and thence employ a steam having a caustic concentration ofabout 2000 parts per million.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The invention has beensummarized above, and will now be explained in terms of certainpreferred embodiments which represent the best mode of the inventionknown to the inventors at the date of this application.

The description of these embodiments of the invention will assume anunderground reservoir containing petroleum oil in formations therein,with a well comrnunicating from the earths surface to said reservoir.

In accordance with a first embodiment of the invention, steam isprovided proximate said well. Such steam may be either dry steam(substantially vapor) or wet steam (mixture of vapor and liquid). Asuitable causticmaterial such as sodium hydroxide is then added to thesteam in a convenient manner so that the concentration of the sodiumhydroxide in the steam is at least 500 parts per million, based on theweight of water used to make steam.

If the steam is wet steam, NaOH in aqueous solution may be added to theliquid phase of the steam. If the steam is dry steam, a separate aqueoussolution of NaOH may be supplied for use therewith.

The steam, containing NaOH, is then injected for a first period of timethrough the well into the underground formation. At the end of the firstperiod of time, injection of steam is tenninated.

Then, for a second period of time, well fluids including oil areproduced through the well. Some amounts of water and steam will beproduced with the well fluids.

The steam injection period and production period constitute the firstcycle of the process in accordance with the invention.

The length of the injection and production periods will depend on theparticular properties of the well and the size and nature of thereservoir, etc. As one example, the first period may be that timerequired for injecting 40,000 barrels of steam, and the second periodmay be that time required for producing any desired number of barrels ofwell fluids, or might be based upon any convenient time period, forexample twice as long as the time of the injection period.

The cycle including steam injection and production may then be repeatedas many times as desired, for example 12.

The NaOH content of the steam injected in this embodiment should be, asmentioned above, at least about 500 parts per million. It is believedthat for most contexts of use the point of diminishing returns isreached at about 6000 parts per million and, in fact, greaterenhancement of recovery begins to lessen at concentrations much above6000 parts per million. Applicants have tested concentrations as high as15,300 parts per million and have found that, under the conditions of 3those tests, enhancement of recovery did occur but was not as pronouncedas such enhancement at about 2000-6000 parts per million. 7

In fact, in many contexts of use, applicants have found that the optimumlevel of NaOI-I content is about 2000 parts per million, with respect toenhancement of recovery of total oil. It is believed that a somewhathigher concentration of about 6000 parts per million is optimum in thecircumstances tested, with respect to initial rate of recovery of oil.

Thus, in accordance with another embodiment of this invention, theconcentration of NaOH in the injected steam is decreased from an initiallevel to about 2000 parts per million, after the first few cycles.Specifically, in accordance with such a preferred embodiment, the NaOl-Icontent is maintained at about 6000 parts per million for the firstcouple of cycles, and then is reduced to about 2000 parts per millionfor several additional cycles. This procedure will result in manycontexts of use, in maximizing both the initial rate of recovery, andthe total recovery of oil in place.

In accordance with yet another embodiment of the invention, steamcontaining caustic is injected for a first period of time, and then thesteam injection is terminated..Following termination of the steaminjection, production from the well is not immediately commenced.Rather, the steam is allowed to soak in the formationfora period oftime. Following a soaking period, then production is commenced and theproduction is continued for a desired period of time. In accordance withthis embodiment, one cycle is represented by the injection period, thesoaking period, and the production period.

I The first cycle may then be repeated as many times as desired, forexample 12. In accordance with this embodiment of the invention, thecaustic concentration of the steam may be similar to that listed in theabove discussion in connection with the former embodiments, and may bedecreased as explained above after the first few cycles.

Applicants have performed several laboratory experiments to simulatefield conditions, and the following examples are taken from theselaboratory experiments.

These examples, which should not be considered as limiting, illustratethe remarkable enhancement of recovery, both in terms of rate ofrecovery and total recovery, as compared to traditional steamstimulation processes.

EXAMPLE I A physical model of a confined quarter five-spot was preparedfor laboratory use and equipped with anexpansion chamber. A single portwas provided for injection and production.

The expansion chamber served to prevent excess ve buildup of pressureduring the injection period. With the expansion chamber, unconfinedfluid movement during the injection period could be accuratelysimulated.

The model was filled with Ottawa quartz sand, and was saturated with aknown amount of oil and water. The oil was crude from the Olig-Potterzone of McKlttrick field in California, which has a viscosity of about17,000 cp at 75F. The water was filtered water from the Kern River areaof California.

Wet steam containing approximately 70% vapor and approximately 30%liquid was provided. This steam was injected into the model for a timeperiod of two 4 minutes. The injection rate was 175.2 co /minute,corresponding to l047barrels/day. The total steam injected during thiscycle totaled an equivalent of 39,600 barrels, or about 7.5% of a modelpore volume.

The steam injection was then terminated, and production of fluids fromthe model was commenced; This production was continued for a period of 4minutes, the fluids produced being cooled through a condenser, andcollected in a container for analysis. Then the production period wasterminated marking the end of a first cycle. The oil and water wererecovered and measured. 11 additional cycles just like the first were,then performed, with the amount of oil and water recovered on each cyclebeing separately collected for measurement.

The production at the end of the-4th, 7th, 10th and 12th cycles wasmeasured. At the end of the twelve cycles, the model was cleaned and theamount of oil remaining in the model was measured from the cleaningfluids. In this experiment and all the others which are enumeratedbelow, the sum of the oil produced and the amount calculated as residualoil was required to be within 2% of the original oil in place, or elsethe results were not considered valid. In each of these examples, suchsum was correct to within 2%. I

The temperature at various points in the model was also measured duringthe experiment, such measurements being made at 16 points at the topandbottom of the model, and at six points in the middle of the model.

The initial oil saturation was measured .at 91.5%.

The results were chartered and the oil recovery at certain timeintervals is reported in Table 1.

EXAMPLE II EXAMPLE In Example I was repeated except that the initial oilsaturation was measured at 97.3%, and 500 parts per million sodiumhydroxide was added to the injected steam.

Measurements like those in Example I were made, and the results areshown in Table l.

The results of this experiment show that 2.77% of the original oil inplace was recovered at the end of 12 cy cles, for a 2% enhancement inproduction relative to the base case.

EXAMPLE IV Example I was repeated, except that the initial oilsaturation was measured at 91.7%, and sodium hydroxide was added to thesteam at a concentration of 2000 parts per million.

Measurements taken at the end of the 4th, 7th, 10th and 12th cycles areindicated inTable l, the total percent of original oil in placerecovered at the end of 12 cycles being 15.3%. This represents a 465%increase relative-to the base case.

EXAMPLE V Example IV was repeated except that the initial oil saturationwas measured at 85. 1%, and the sodium hydroxide concentration was 4000parts per million.

In this example, as seen from Table 1, the percent of original oil inplace recovered at the end of the 12th cycle was 13.7. This represents a410% improvement relative to the base case.

EXAMPLE VI Example IV was repeated except that the initial oilsaturation was measured at 87.6%, and the sodium hydroxide concentrationwas 6000 parts per million. As seen from Table 1, 12.2% of the originaloil in place was recovered at the end of the 12 cycles. This representsa 350% enhancement relative to the base case.

EXAMPLE VII EXAMPLE VIII Example IV was repeated except that the initialoil saturation was measured at 89.5%, and the sodium hydroxideconcentration was 15,300 parts per million.

As seen from Table l, 9.4% of the original oil in place had beenrecovered at the end of the 12 cycles, representing a 250% increase inproduction relative to the base case.

tion level for these examples is less than 15,300 parts per million.

In other contexts of use, caustic levels considerably higher than thoseindicated above might be employed. For example, it is believed that insome contexts of use, caustic levels as high as about 50,000 parts permillion might be advantageously employed. The desired caustic contentwill depend on the physical and chemical properties of the field crude,and the reservoir material, and can be determined for each context ofuse by laboratory experiment or field trial.

Optimization of the sodium hydroxide content will vary from case to casebut in most instances for situations similar to these examples it isbelieved that such optimum concentration would be, based upon alaboratory model, from about 2000 to about 6000 parts per million.

Because of the relatively low chemical adsorption and reactivity of thesand used in the laboratory model, it is believed that sodium hydroxideconcentration for use in field applications will need to be considerablygreater than those used in the laboratory models. Thus, it is believedfor field applications of these examples, the optimum concentration ofsodium hydroxide would be somewhere between 2000 parts per million and15,000 parts per million.

Since rather remarkable enhancement of the recovery of the original oilin place was noted at concentrations up to 15,300 parts per million,applicants believe that it would be prudent in most instances to err onthe side of too much rather than too little, sodium hydroxide. This biaswill of course be weighed against the availability and cost of sodiumhydroxide or other suitable caustic material.

The temperature measurements from Examples I-VIII indicated that whenthe injected concentration of NaOH was 2000 parts per million orgreater, the recorded temperatures in the lower portion of the modelwere higher and more uniform than when lower con- TABLE 1 FinalPercentage Increase in NaOH Initial Oil Production Example ConcentrationSaturation Percent of 001? Recovered at: Relative to No. ppm 24 min 42min min 72 min Base Case 1 0.0 91.5 0.26 0.90 2.20 2.70 II 0.0 89.7 0.330.86 1.80 2.69 I11 500 97.3 0.42 1.41 2.27 2.77 2 1V 2,000 91.7 8.8 11.513.6 15.3 465 V 4,000 85.1 7.3 10.7 12.9 13.7 410 V1 6,000 87.6 9.8 11.411.9 12.2 350 Vll 6,000/2,000 87.0 11.4 14.5 16.2 17.1 530 Vlll 15,30089.5 3.3 5.7 8.5 9.4 250 As seen from the results tabulated in Table l,the best results in these laboratory studies were obtained by using 6000parts per million sodium hydroxide for the first two cycles of the steamstimulation process, and 2000 parts per million for the latter 10cycles. However, dramatic improvements were shown in each of ExamplesIV, V, VI, VII, and VIII. The improvement in Example VIII, wherein thesodium hydroxide concentration was 15,300 parts per million was lessdramatic than in the other examples. This fact, coupled with the extentof the additional sodium hydroxide required to achieve the concentrationlevel of Example Vlll indicates that the optimum sodium hydroxideconcentracentrations of sodium hydroxide were employed.

At the time of this application, it is believed that the preferredcaustic material for most contexts of use is sodium hydroxide.Consequently, the above discussion and all of the above examples havebeen in terms of NaOH. It is applicants belief, however, that other suitable caustic materials may be alternatively employed, for examplepotassium hydroxide, sodium carbonate, and mixtures of the same, ormixtures of sodium hydroxide with either or both of these materials.

It is well known that the art is continually seeking better ways toenhance production of oil, and many of these ways have included the useof steam either in steam stimulation processes or steam flooding pro- 7cesses. In Attachment A, which is attached hereto and made a part ofthis application, applicants have enumerated a number of citations whichthey consider pertinent to some degree or another in connection withthis invention.

In the copending application of Shen, Ser. No. 367,571, filed June 6,1973, and now U.S. Pat. No. 3,853,178, methods for improving recoveryare disclosed in connection with steam flooding processes performed inthe context of an injection well and a producing well, wherein smallamounts of sodium hydroxide may be added to the steam. In thatapplication, the amounts of NaOH are, in general, even smaller thanthose added in this invention.

Although the invention has been described in terms of particularpreferred embodiments and examples, it will be apparent to those ofskill in the art that various changes may be made in the processesdescribed without departing from the scope of the invention.

What is claimed is:

1. A method for enhancing the recovery of oil from an undergroundreservoir having a well communicating thereto, comprising:

providing a source of steam for said well;

adding caustic material to said steam so that the concentration of saidcaustic material in said steam is in excess of 500 parts per million;

injecting said steam containing said caustic material into saidreservoir through said well for a first period of time;

terminating the injection of'said steam, and thence commencing theproduction of well fluids from said well;

continuing the production of well fluids for a second period of time,said first period and said second period defining a steam stimulationcycle; terminating said production period; and, repeating said steamstimulation cycle several times. 2. The method of claim 1 wherein saidconcentration of caustic material is in excess of 2000 parts permillion.

3. A method for enhancing the recovery of oil from an undergroundreservoir having a well communicating thereto, comprising:

providing a source of steam for said well; adding caustic material tosaid steam so that the concentration of said caustic material in saidsteam is between about 2000 parts per million and about 15,000 parts permillion;

injecting said steam containing said caustic material into saidreservoir through said well for a first period of time;

terminating the injection of said steam, and thence commencing theproduction of well fluids from said well;

continuing the production of well fluids for a second period of time,said first period and said second period defining a steam stimulationcycle; terminating said production period; and,

repeating said steam stimulation cycle several times.

4. The method of claim 3, wherein the length of said second period isapproximately twice the length of said first period.

5. The method of claim 3, wherein the steam injected during said firstperiod of each cycle is equal to approximately 7.5% of the pore volumeof said reservoir.

6. The method of claim 3, wherein twelve steam stimulation cycles areemployed.

8 7. The method of claim 3, wherein said caustic material is sodiumhydroxide.

8. The method of claim 7, wherein said sodium hydroxide concentration isabout 6000 parts per million. 9. A method for enhancing the recovery ofoil from an underground reservoir having a well communicating thereto,comprising:

providing a source of steam for said well; adding caustic material tosaid steam so that the concentration of said caustic material in saidsteam is at least approximately 6000 parts per million;

injecting said steam containing said caustic material into saidreservoir through said well for a first period of time;

terminating the injection of said steam, and thence commencing theproduction of well fluids from said well;

continuing the production of well fluids for a second period of time,said first period and said second period defining a steam stimulationcycle; terminating said production period;

repeating said steam stimulationcycle at least one more time;

providing additional steam for said well;

adding caustic material to said steam so that the concentration of saidcaustic material in said steam is approximately one-third that employedduring the first steam stimulation cycle;

injecting said steam containing caustic material into said reservoirthrough said well for a period of time similar to said first period oftime;

terminating the injection of said steam, and thence commencing theproduction of well fluids from said well;

continuing the production of well fluids for a further period of timesimilar to said second period of time, said further injection andproduction periods defining a further steam stimulation cycle;

terminating said production period; and,

repeating said further steam stimulation cycle several times.

10. The method of claim 9, wherein the length of said second period isapproximately twice the length of said first period.

11. The method of claim 9, wherein the steam injected during said firstperiod of each cycle is equal to approximately 7.5% of the pore volumeof said reser- VOII'.

12. The method of claim 9, wherein twelve steam stimulation cycles areemployed.

13. The method of claim 9, wherein said caustic material is sodiumhydroxide.

14. A method for enhancing the recovery of oil from an undergroundformation having a well communicating thereto, comrising:

providing a source of steam for said well;

adding caustic material to said steam so that the concentration of saidcaustic material in said steam is in excess of 500 parts per million;

injecting said steam containing caustic material into said reservoirthrough said well, for a first period 0 time; terminating the injectionof said steam, and thence allowing the steam to soak into the formationfor a second period of time;

thence commencing production of well fluids from said well, andcontinuing such production for a third period of time;

9 said first, second and third periods of time defining a steamstimulation cycle;

thence terminating production from said well; and,

repeating said steam stimulation cycle several times.

15. The method of claim 14, wherein said concentration of causticmaterial is in excess of 2000 parts per million.

16. The method of claim 14, wherein the concentration of said causticmaterial in said steam is between about 2000 parts per million and about15,000 parts per million.

17. The method of claim 16, wherein the length of said third period isapproximately twice the length of said first period.

18. The method of claim 16, wherein the steam injected during said firstperiod of each cycle is equal to approximately 7.5% of the pore volumeof said reser- 19. The method of claim 16, wherein twelve steamstimulation cycles are employed.

20. The method of claim 16, wherein said caustic ma terial is sodiumhydroxide.

21. The method of claim 20, wherein said sodium hydroxide concentrationis approximately 6000 parts per million.

22. The method of claim 21, wherein said sodium hydroxide concentrationis reduced to about 2000 parts per million after the first few cycles.

23. The method of claim 22, wherein the length of said third period isapproximately twice the length of said first period.

24. The method of claim 22, wherein the steam injected during said firstperiod of each cycle is equal to approximately 7.5% of the pore volumeof said reservoir.

25. The method of claim 22, wherein twelve steam stimulation cycles areemployed.

1. A METHOD FOR ENHANCING THE RECOVERY OF OIL FROM AN UNDERGROUNDRESERVOIR HAVING A WELL COMMUNICATING THERETO, COMPRISING: PROVIDING ASOURCE OF STEAM FOR SAID WELL; ADDING CAUSTIC MATERIAL TO SAID STEAM SOTHAT THE CONCENTRATION OF SAID CAUSTIC MATERIAL IN SAID STEAM IS INEXCESS OF 500 PARTS PER MILLION; INJECTING SAID STEAM CONTAINING SAIDCAUSTIC MATERIAL INTO SAID RESERVOIR THROUGH SAID WELL FOR A FIRSTPERIOD OF TIME; TERMINATING THE INJECTION OF SAID STEAM, AND THENCECOMMENCING THE PRODUCTION OF WELL FLUIDS FROM SAID WELL; CONTINUING THEPRODUCTION OF WELL FLUIDS FOR A SECOND PERIOD OF TIME, SAID FIRST PERIODAND SAID SECOND PERIOD DEFINING A STEAM STIMULATION CYCLE; TERMINATINGSAID PRODUCTION PERIOD; AND, REPEATING SAID STEAM STIMULATION CYCLESEVERAL TIMES.
 2. The method of claim 1, wherein said concentration ofcaustic material is in excess of 2000 parts per million.
 3. A method forenhancing the recovery of oil from an underground reservoir having awell communicating thereto, comprising: providing a source of steam forsaid well; adding caustic material to said steam so that theconcentration of said caustic material in said steam is between about2000 parts per million and about 15,000 parts per million; injectingsaid steam containing said caustic material into said reservoir throughsaid well for a first period of time; terminating the injection of saidsteam, and thence commencing the production of well fluids from saidwell; continuing the production of well fluids for a second period oftime, said first period and said second period defining a steamstimulation cycle; terminating said production period; and, repeatingsaid steam stimulation cycle several times.
 4. The method of claim 3,wherein the length of said second period is approximately twice thelength of said first period.
 5. The method of claim 3, wherein the steaminjected during said first period of each cycle is equal toapproximately 7.5% of the pore volume of said reservoir.
 6. The methodof claim 3, wherein twelve steam stimulation cycles are employed.
 7. Themethod of claim 3, wherein said caustic material is sodium hydroxide. 8.The method of claim 7, wherein said sodium hydroxide concentration isabout 6000 parts per million.
 9. A method for enhancing the recovery ofoil from an underground reservoir having a well communicating thereto,comprising: providing a source of steam for said well; adding causticmaterial to said steam so that the concentration of said causticmaterial in said steam is at least approximately 6000 parts per mIllion;injecting said steam containing said caustic material into saidreservoir through said well for a first period of time; terminating theinjection of said steam, and thence commencing the production of wellfluids from said well; continuing the production of well fluids for asecond period of time, said first period and said second period defininga steam stimulation cycle; terminating said production period; repeatingsaid steam stimulation cycle at least one more time; providingadditional steam for said well; adding caustic material to said steam sothat the concentration of said caustic material in said steam isapproximately one-third that employed during the first steam stimulationcycle; injecting said steam containing caustic material into saidreservoir through said well for a period of time similar to said firstperiod of time; terminating the injection of said steam, and thencecommencing the production of well fluids from said well; continuing theproduction of well fluids for a further period of time similar to saidsecond period of time, said further injection and production periodsdefining a further steam stimulation cycle; terminating said productionperiod; and, repeating said further steam stimulation cycle severaltimes.
 10. The method of claim 9, wherein the length of said secondperiod is approximately twice the length of said first period.
 11. Themethod of claim 9, wherein the steam injected during said first periodof each cycle is equal to approximately 7.5% of the pore volume of saidreservoir.
 12. The method of claim 9, wherein twelve steam stimulationcycles are employed.
 13. The method of claim 9, wherein said causticmaterial is sodium hydroxide.
 14. A method for enhancing the recovery ofoil from an underground formation having a well communicating thereto,comrising: providing a source of steam for said well; adding causticmaterial to said steam so that the concentration of said causticmaterial in said steam is in excess of 500 parts per million; injectingsaid steam containing caustic material into said reservoir through saidwell, for a first period of time; terminating the injection of saidsteam, and thence allowing the steam to soak into the formation for asecond period of time; thence commencing production of well fluids fromsaid well, and continuing such production for a third period of time;said first, second and third periods of time defining a steamstimulation cycle; thence terminating production from said well; and,repeating said steam stimulation cycle several times.
 15. The method ofclaim 14, wherein said concentration of caustic material is in excess of2000 parts per million.
 16. The method of claim 14, wherein theconcentration of said caustic material in said steam is between about2000 parts per million and about 15,000 parts per million.
 17. Themethod of claim 16, wherein the length of said third period isapproximately twice the length of said first period.
 18. The method ofclaim 16, wherein the steam injected during said first period of eachcycle is equal to approximately 7.5% of the pore volume of saidreservoir.
 19. The method of claim 16, wherein twelve steam stimulationcycles are employed.
 20. The method of claim 16, wherein said causticmaterial is sodium hydroxide.
 21. The method of claim 20, wherein saidsodium hydroxide concentration is approximately 6000 parts per million.22. The method of claim 21, wherein said sodium hydroxide concentrationis reduced to about 2000 parts per million after the first few cycles.23. The method of claim 22, wherein the length of said third period isapproximately twice the length of said first period.
 24. The method ofclaim 22, wherein the steam injected during said first period of eachcycle is equal to approximately 7.5% of the poRe volume of saidreservoir.
 25. The method of claim 22, wherein twelve steam stimulationcycles are employed.